1. Field of the Invention
The present invention relates generally to variable valve timing arrangements for internal combustion engines and more specifically to a control arrangement via which a cam shaft cam be selectively moved in its axial direction.
2. Description of the Prior Art
FIG. 13 shows a previously proposed arrangement which was disclosed in the May edition of the MTZ (Motortechnische Zeitschrift). As shown, this arrangement is applied to a direct drive type overhead cam arrangement which obviates the need for rocker arms by arranging for the tops of the valve lifters or tappets 1 to directly engage with their respective motivating cams 2.
In the illustrated arrangement, flat plate plate-like members 4 are pivotally disposed at the tops of the lifters 1 and arranged to act as cam followers. The cams 2 are formed on a cam shaft 6 which is selectively displaceable in its axial direction by the actuator arrangement generally denoted by the numeral 8.
The actuator 8 is disposed within a hollow drive pulley 10 which is operatively connected with the engine crankshaft of the engine by way of a cogged belt (neither shown).
One end of the cam shaft 6 is splined and arranged to be received in a correspondingly internally splined sleeve-like cylinder 12. A variable volume control chamber 14 is defined in the cylinder by the splined end of the cam shaft 6. The cylinder 12 is connected with the body of the pulley 10 so as to undergo synchronous rotation therewith and is formed with a supply port 16 via which the control chamber 13 fluidly communicates with a non-illustrated source of hydraulic fluid under pressure.
The cam shaft is formed with a coaxial bore 18 via which lubricating hydraulic fluid is supplied to the surfaces of the cams 2 and the like which undergo relative rotation with respect to one another.
The actuator 8 includes an exhaust port 20 via which the hydraulic fluid which is introduced into the control chamber 14 is controllably drained therefrom. A seal 22 is provided on the cam shaft adjacent the splining and arranged to sealingly engage the inner surface of the cylinder. A stopper 24 is provided on the cam shaft. This stopper is arranged to engage a flange 26 formed at the end of the cylinder 12 and thus limit the degree to which the cam shaft 6 may slide into the cylinder 12.
By controlling the amount of hydraulic fluid which is supplied to the supply port 16, the amount of fluid which is retained in the chamber 14 and the amount of axial displacement of the cam shaft 6 is controlled.
The splines formed on the cam shaft and in the cylinder are arranged to induce a predetermined amount of relative rotation between the cam shaft 6 and the sleeve 12 as the cam shaft 6 is axially displaced.
The cams 2 have an essentially frusto-conical configuration and are arranged in this instances so that they taper in the direction of the actuator 8.
This configuration, in combination with the relative rotation which occurs between the cam shaft and the sleeve-like cylinder permits both the amount and the timing of the lift which is induced in the valves, to be varied in accordance with the amount of axial displacement of the cam shaft.
However, this arrangement has encountered a drawback in that it is difficult to appropriately control the amount of displacement over a wide range of engine operating conditions.
For example, it is required that large amounts of torque be produced when the engine throttle valve is wide open, while promoting efficient combustion which reduces noxious emissions under partial load-medium/low engine speed modes of operation.